KUNDUZ, Afghanistan (Reuters) - An Afghan man killed his wife for giving birth to a third daughter rather than the son he hoped for, police in Afghanistan's northern Kunduz province said on Monday.

The victim, 28, known by the one name of Storai, was strangled by her husband -- a local militia member -- and his mother on Saturday in revenge for bearing the couple's third daughter three months ago in Mohasili village, police said.

Police said they arrested the victim's mother-in-law in connection with her death, but Storai's husband was still at large, likely sheltered by heavily-armed militia colleagues.

"The existence of militiamen is a huge problem and therefore we face difficulty in arresting him," said Kunduz police chief Sufi Habib.

Nadera Geya, head of the Kunduz women's affairs department, called the killing one of the worst examples of violence against women she had encountered.

Violence against women is commonplace in Afghanistan. In late November in the same province, an Afghan family that refused to give their daughter in marriage to a man they considered irresponsible was attacked at home by assailants who poured acid over both parents and three children.

Police later arrested the rejected suitor and his three brothers for the attack.

With foreign combat troops set to leave Afghanistan by the end of 2014, and moves ongoing to kickstart a peace process involving the ultra-conservative Taliban, rights watchdogs inside and outside Afghanistan fear women's rights may be sacrificed.

"The rights of women cannot be relegated to the margins of international affairs, as this issue is at the core of our national security and the security of people everywhere," the U.S. embassy in Kabul said in a statement on Monday.